1. Field of the Invention
The invention relates generally to data communications and, more particularly, to reducing inter-symbol interference in data communications.
2. Discussion of the Related Art
This section is intended to introduce aspects that may be helpful to facilitating a better understanding of aspects of the invention. Accordingly, statements of this section are to be read in this light and are not to be understood as admissions about the scope of the prior art.
Data communication schemes have handled inter-symbol interference (ISI) by a variety of techniques. One such technique is known as orthogonal frequency division multiplexing (OFDM). OFDM uses modulation waveforms that enable the essential removal of ISI in a frequency-dependent channel.
In OFDM, each transmitted data block is a weighted superposition of OFDM modulation waveforms. The OFDM modulation waveforms form an orthonormal basis set over a time period (TS-Tg) where TS is the length of the OFDM block and Tg is the duration of either a guard interval or a cyclic prefix. The communication channel has a delay spread TD that is the length of a communication channel's memory. Since ISI does not distort symbols separated by the communication channel's delay-spread, Tg is selected to be greater than or equal to TD in OFDM. In an OFDM block, the weights of the superposition define the data symbol being transmitted.
In OFDM, each transmitted data block is demodulated by projecting the received data block onto a basis set of conjugate OFDM modulation waveforms. Since the OFDM modulation waveforms are a basis set over the interval (TS-Tg), the projections may be performed over the last (TS-Tg) of the OFDM data blocks. That is, the projections do not need to use prefix portions of the OFDM data blocks. Since the channel memory is limited to a time of length TD, an earlier transmitted OFDM block substantially only produces ISI in the cyclic prefix or guard portion of the next received OFDM data block. Thus, by ignoring said cyclic prefix or guard portions of received OFDM data blocks, OFDM produces demodulated data that is substantially free of distortion due to ISI. OFDM techniques may also effectively diagonalize the communication channel.
Unfortunately, cyclic prefix and guard portions of OFDM data blocks consume bandwidth that might otherwise be used to transmit data. As the communication channel's delay-spread approaches the temporal length of the OFDM data block, the bandwidth remaining for carrying data shrinks to zero. Thus, OFDM communication schemes would not be expected to be bandwidth-efficient in communication channels whose delay-spreads approach the length of the OFDM data blocks.